1. Field of the Invention
The present invention relates to a method and apparatus for determining which samples of a plurality of samples include one or more of a class of target materials. More specifically, the present invention relates to a method and apparatus which pre-screens the samples to determine which samples produce a piezoelectric resonance signal indicating the potential presence of the target material. For each sample producing the piezoelectric resonance signal, an additional detection process, such as nuclear quadrupole resonance, nuclear magnetic resonance, x-ray, canine, or manual inspection can be used to confirm the presence of the target material.
2. Description of the Related Art
There are many situations where it is desirable to detect the presence of a target material (that is, a specific substance). For example, with the unfortunate increase in drug trafficking and terrorist use of high explosives in aircraft and buildings, detection systems are often used to detect sub-kilogram quantities of narcotics and explosives against a background of more benign materials. For example, such a detection system currently is being used in airports on an experimental basis to detect narcotics or explosives hidden in luggage.
Nuclear quadrupole resonance (NQR) is a known technique for detecting a target material. Generally, radio frequency (RF) radiation at a particular frequency will induce a detectable precession in nuclei in specific substances, but not in other substances. Nuclear quadrupole resonance (NQR) takes advantage of this phenomenon to detect one of these specific substances as a target material.
FIG. 1 is a diagram illustrating a conventional NQR apparatus. Referring now to FIG. 1, a transmitter 20 and a receiver 22 are connected to a probe 24 through a transmit/receive (T/R) switch 26. Probe 24 includes a coil (not illustrated) forming part of a resonant circuit (not illustrated). To detect the presence of a target material, T/R switch 26 connects transmitter 20 to probe 24 while disconnecting receiver 22 from probe 24. Then, transmitter 20 generates a pulse and supplies the pulse to probe 24. Generally, the pulse is formed by a signal having a frequency corresponding to the resonance signal of the nuclei of the target material which is intended to be detected. Probe 24 receives the pulse, which causes the coil in probe 24 to store (RF) energy. If a sample (not illustrated) is appropriately placed near the coil, the stored RF energy will cause a corresponding electromagnetic field to irradiate the sample. If the sample includes the target material, the electromagnetic field may induce a nuclear quadrupole resonance signal in the target material.
After the sample is irradiated with the electromagnetic field, T/R switch 26 connects receiver 22 to probe 24 while disconnecting transmitter 20 from probe 24. The coil in probe 24 then detects the nuclear quadrupole resonance signal from the target material, and probe 24 produces a corresponding output signal. The output signal of probe 24 is received and analyzed by receiver 22, to confirm the presence of the target material in the sample.
NQR detection systems for the detection of explosives and narcotics are disclosed in U.S. patent titled “DETECTION OF EXPLOSIVE AND NARCOTICS BY LOW POWER LARGE SAMPLE VOLUME NUCLEAR QUADRUPOLE RESONANCE (NQR)”, U.S. Pat. No. 5,233,300; U.S. patent titled “REMOVING THE EFFECTS OF ACOUSTIC RINGING AND REDUCING TEMPERATURE EFFECTS IN THE DETECTION OF EXPLOSIVES BY NQR”, U.S. Pat. No. 5,365,171; U.S. patent titled “DETECTION OF EXPLOSIVES BY NUCLEAR QUADRUPOLE RESONANCE”, U.S. Pat. No. 5,206,592, and “A MEANS FOR DETECTING EXPLOSIVES AND NARCOTICS BY STOCHASTIC NUCLEAR QUADRUPOLE RESONANCE (NQR)” U.S. Pat. No. 5,608,321 which are incorporated herein by reference.
Nuclear magnetic resonance (NMR) is an additional, known technique for detecting a target material. In NMR, a sample is placed in a static (or constant) magnetic field, and irradiated with rf magnetic field as in NQR. If the sample includes the target material, the rf field, under the right conditions, will cause a nuclear magnetic resonance signal to be induced in the target material. The nuclear magnetic resonance signal is then detected, to indicate the presence of the target material in the sample. For example, an NMR detection system is disclosed in U.S. patent titled “SYSTEM FOR DETECTING NUCLEAR MAGNETIC RESONANCE SIGNALS FROM SMALL SAMPLES”, U.S. Pat. No. 5,166,615, which is incorporated herein by reference.
Therefore, FIG. 1 can also be used to describe a conventional NMR apparatus, with the addition of a magnet (not shown). Referring now to FIG. 1, to induce a magnetic resonance signal in a sample, T/R switch 26 connects transmitter 20 to probe 24 while disconnecting receiver 22 from probe 24. Then, transmitter 20 generates a pulse and supplies the pulse to probe 24 in the static magnetic field. Probe 24 receives the pulse, and produces a corresponding rf magnetic field. A sample (not illustrated) is then placed near probe 24 to be irradiated with the rf magnetic field. If the sample includes the target material, the rf magnetic field may induce a nuclear magnetic resonance signal in the target material.
After the sample is irradiated with the magnetic field, T/R switch 26 connects receiver 22 to probe 24 while disconnecting transmitter 20 from probe 24. Probe 24 then detects the nuclear magnetic resonance signal induced in the target material, and probe 24 produces a corresponding output signal. The output signal of probe 24 is received and analyzed by receiver 22, to confirm the presence of the target material in the sample. Multiple repetitions (scans) are also possible. Therefore, NMR is similar to NQR in that a resonance signal is induced in the target material, and the resonance signal is then detected. Unfortunately, such apparatuses are expensive and the weak NQR/NMR signals may provide slow detection of a target material.